FFT from a excel file

I want to compute FFT on a set of data I have recorded.

Here is the code I am using and the error I am getting:

>> xdft = fft(channel1Data2(:,2));

Error using fft

Invalid data type. First argument must be double,

single, int8, uint8, int16, uint16, int32, uint32,

or logical.

[Mathieu NOE]

what is the type and dimension of channel1Data2 ?

[Arnav Dabak]

The dimension of the data is 1000002 x 2,

Column 1 is Time and Column2 is Amplitude.

Answer 1

[MarKf]

Well, make sure that the data contained in channel1Data2 is numeric (double, single, int8, etc..).

From the question title it seems that it is obtained by reading an Excel file, so likely the format/type has been messed up there. You could change the format to double with for example str2double (if the obtained data type happens to be a string), do check what's in that variable though (famous Excel® issue of converting values to nonsensical dates).

[Arnav Dabak]

I checked the data type and it is double and I am reading it from an excel file.

I have attached a screenshot of the error.

Answer 2

[Star Strider]

EDIT — (1 Jun 2023 at 16:07)

The array is table apparently created by readtable. Use cell array indexing (explained below) or refer to it as:

xdft = fft(channel1Data2.Ampl);

or:

xdft = fft(channel1Data2.{:,2});

The cell array indexing approaches will also work, so I will leave that part of my original Answer unchanged.

—

It could be that ‘channel1Data2’ is a cell array. If so, converting it to a numeric array should work —

channel1Data2 = num2cell(rand(10,2))

channel1Data2 = 10×2 cell array
    {[0.4233]}    {[0.9203]}
    {[0.8342]}    {[0.4878]}
    {[0.0641]}    {[0.7609]}
    {[0.4819]}    {[0.8762]}
    {[0.2812]}    {[0.1188]}
    {[0.5806]}    {[0.8916]}
    {[0.7699]}    {[0.4074]}
    {[0.9036]}    {[0.8791]}
    {[0.9735]}    {[0.3604]}
    {[0.6840]}    {[0.7983]}

xdft = fft(cell2mat(channel1Data2(:,2)));               % Use 'cell2mat'
xdft

xdft = 
   6.5008 + 0.0000i
   0.4475 - 0.0261i
   0.0448 - 0.2162i
   0.3068 + 0.8035i
   1.2347 + 0.3965i
  -1.3651 + 0.0000i
   1.2347 - 0.3965i
   0.3068 - 0.8035i
   0.0448 + 0.2162i
   0.4475 + 0.0261i

xdft = fft([channel1Data2{:,2}]).';                     % Cell Array Addressing (With Subsequent Transpose)
xdft

xdft = 
   6.5008 + 0.0000i
   0.4475 - 0.0261i
   0.0448 - 0.2162i
   0.3068 + 0.8035i
   1.2347 + 0.3965i
  -1.3651 + 0.0000i
   1.2347 - 0.3965i
   0.3068 - 0.8035i
   0.0448 + 0.2162i
   0.4475 + 0.0261i

xdft = fft(vertcat(channel1Data2{:,2}));                % Cell Array Addressing (With 'vertcat')
xdft

xdft = 
   6.5008 + 0.0000i
   0.4475 - 0.0261i
   0.0448 - 0.2162i
   0.3068 + 0.8035i
   1.2347 + 0.3965i
  -1.3651 + 0.0000i
   1.2347 - 0.3965i
   0.3068 - 0.8035i
   0.0448 + 0.2162i
   0.4475 + 0.0261i

xdft = fft(channel1Data2(:,2));                         % Numeric Array Addressing Of 'cell' Array (Reproduces The Error)

Error using fft
Invalid data type. First argument must be double, single, int8, uint8, int16, uint16, int32, uint32, or logical.

xdft

So there are several ways to solve this. Choose one.

.